While in the world, I saw a new figure on the street with me. I clicked on it and it claimed to be an Extremely Rare Items Vendor.
It had five items for sale:
Rube Shaped Potato
Million Currant Trophy
Factory Defect Chick Cubimal
Highest Quality #1 "Imported" Glitchmas Yeti
each item on its info screen had a limited quantity (999 Antique Spigots will ever be in the game) decreasing in number as they increased in cost (the Yeti was most expensive and will have the fewest available(300 total)).
I was only able to afford the two cheapest items. I'm not going to list costs or where I found this mysterious vendor so that other people can have an experience as intended!!
I see (thanks to the Scarf s'reference) the collectibles are the "suspucious" shop items that OP describe,easter eggs,gems,unique items (like dust stick,ultimate beta item ((or S.I.T.O.B.T.G)),trophyies,music box & cubimals...ect
Well, I know one thing. If that "vendor" wants me to keep quiet about that contraband he and his friends had me run for them the other day, he'll do the right thing and fork over some of that stash he's got. If not, I might just have a chat with a lizard or two over at Bureaucratic Hall. And time's running out...